Check my Maths LOL!
Turbo Mini Forums

So with Castle Combe approaching quickly and the weather looking good, the task in hand at Spannerbitch HQ is preparing the DON for some circuit use.

The main stumbling block is the gearing, but there is bugger all I can do about that so make sure you wave when you pass me halfway down the straights sat on the rev limiter, and I promise to wave back when I hurtle back past coming out of the slower parts....

Apart from that the roll cage is going back in, and I've mounted a second seat so our gert can come for a ride round with me.

All the front suspension is adjustable so I have that to check, although I am not planning on moving anything apart from the damper settings, however the rear suspension has 350lb springs in at the moment! So I am planning on something softer.
The rear springs are mounted at 30 degrees to the vertical, therefore the rate at the wheel is less than the spring rate, the mount on the trailing arm is near enough in line with the stub axle.

My calculations mean that the effective rate at the rear arm with the 350lb springs is 303lbs, the other springs I have to use are 80lb springs which would give 69lbs at the arm, no use at all!
From the discussion thread started by Robert a few weeks back, taking into account all the replies, I am aiming for 140lbs spring rate at the arm, calculating this back means I need a 160lb spring.

Have I done this right or am I talking bollocks LOL! Plus any thoughts on the spring rate? I know Scruffy uses 150lb springs, but his car is heavier than The DON, hence considering 140lbs.

Maths looks ok to me:
350 x cos 30 = 303
140 / cos 30 = 160

I'm no help on the spring rate though

the maths is correct indeed

I've seen the future and tbh its Pie

If you can tell me your corner weights I will run it through my spreadsheet and tell you what it comes up with.
It's based on Alan Staniforth's info (Terrapin) if you remember that
Regards
Dave

Unfortunately Dave the car hasn't been on a set of 4 wheel scales, its only been on the weighbridge at work where the entire thing weighed 440kgs.
If I get there in time on Friday I hope to get it on my mates 4 wheel scales and see what is what.

OK. Let's make a guess at front/rear distribution and I will see what it comes up with. 70/30 or 60/40? you decide
Regards
Dave

70/30 most definately, there is no weight to the back of the car at all, all that is there is the alloy rear arms bolted direct to the heelboard, alloy hubs, ti stub axles, ti centre locks, fuel pump and filter, alloy tank.

I didn't know you could get springs in 10lb increments?

What i've seen thus far in this post doesn't seem out of the question. sure - maybe you'll find that with the first set of springs you buy you'll need to go one harder or one softer - but they aren't going to break the bank?

Any chance you can move the mounting points for fine tuning?

Making a few sensible assumptions the rate of the spring came out at 170 lb/in.
If you need a more accurate figure I will need more info.
It looks as if you are in the ballpark with your original assumption.
Regards
Dave

Well in the end the choice was dictated by availability; you are right Dave (turbo) they are not available in 10lb increments in 1.9" I/D, they are available in 10lb increments in 2.25" I/D though, this must have been what I had in mind.

So anyway, there were no 150lbs in stock, so I went for a pair of 175lb springs, Dave (danboy) is the 170lb/in your spreadsheet came up with the rate at the wheel, or the rate of the spring taking into account the geometry (e.g. 30 degrees to vertical).
I suppose the mounting points could be moved if it came to it, I'd be reluctant to do so though really, after all the car is 1/4 mile orientated, all the above is really just in order to make a "best guess" at a spring rate to go and have a play around at Castle Combe next weekend, they will be coming back off the car after that for the 350's to go back on.

Tom it was actual spring rate
Regards
Dave

Great stuff, it seems my educated guess/buying what was available in buying 175lb springs isn't a million miles away!

Thank you for running the sums Dave!